Modern municipal sewage waste treatment plants utilize conventional mechanical and biological processes to reclaim wastewaters in a process which has an overall effect of converting a water pollution problem into a solid waste disposal problem (sludges). In a typical modern treatment plant the large objects and the grit are separated. Then the wastewater goes to primary sedimentation tanks, which remove 50%-70% of the suspended solids and 25%-40% of the BOD. This sludge and the ground screenings then are subjected to anaerobic digestion. The wastewater then flows to aeration tanks, where the colloidal and dissolved organic matter are converted into gases (primarily carbon dioxide) and cell mass by the aerobic growth of microorganisms, principally bacteria. The cell mass is removed in a secondary sedimentation step and sent to anaerobic digestion. Increasingly, a final biological step reduces the nitrogen content of the effluent by converting the ammonia to nitrate and then to nitrogen gas. This process also produces a biological sludge. The combined sludges are thickened to increase their solids content and sent to anaerobic digestion. The primary purposes of digestion are to reduce the organic content, volume, and odor potential of the sludge, and to reduce the concentration of pathogenic microorganisms (Metcalf & Eddy 1979, U.S. EPA 1979, Arora 1980, Federal Register 1989). The dewatered sludge from anaerobic digestion is the largest solid output from sewage treatment plants and presents the major disposal problem.
The disposal of microbial sludge solids resulting from conventional municipal sewage treatment historically has been expensive because of the extremely large volumes with which these sludges are produced. These sludges contain high fractions of volatile solids (VS), and retain large amounts of water (70-85% before drying). Because of the substantial bulk of the waste, transportation and disposal costs are significant. Recently, the costs for disposal of these microbial sludges through conventional landfilling has risen dramatically because of decreasing landfill availability. In some areas microbial sludges are banned altogether from the landfill because of their high pollution potential.
In light of rising costs for disposing sewage derived microbial sludges, much effort has been expended into alternatives to landfill disposal such as technology which may further reduce the water content of sludges in order to reduce the bulk of the waste requiring disposal. Public concern over possible hazardous emissions through combustion processes and possible heavy metal contamination from the resulting ash has reduced acceptance of combustion (Samela, et al. Environmental Aspects of the Combustion of Sewage Sludge in a Utility Boiler, Environ. Progress, 5:110, 1986) as a disposal option for municipal sewage sludges. The land application of sewage sludge is also problematic as biological activity produces methane and residual volatile solids result in organics contaminating groundwaters.
The cost of disposing of a given amount of sludge is often high and is growing higher. Further, increased loads on existing treatment plants also lead to sharply higher disposal costs. Increasing environmental requirements on the quality of wastewater treatment have resulted in a more complex process which produces greater microbial biomass for disposal. See Laughton, P. J., “Upgrading a Water Pollution Control Plant to Meet Stringent Effluent Discharge Requirements”, Water and Pollution Control, 117:14 (1979). The greater organic loading of wastewater streams has created a higher stress on the treatment process that often reduces the organic removal efficiency. See Mungsgaard et al., “Flow and Load Variations at Wastewater Treatment Plants”, J. Water Pollution Control Fed., 52:2131 (1980).
This reduced efficiency degrades the sludge's dewatering properties, substantially increasing the water content and volume of the waste. See Rutherford et al., “Realities of Sludge Dewatering”, Proceedings of the National Conference on Municipal Treatment Plant Sludge Management, Orlando, Fla. (1986). Finally, reduced dewatering efficiency requires increased use of organic polymers to facilitate dewatering. See Novak et al., “Mixing Intensity and Polymer Performance in Sludge Dewatering”, J. Environ. Engineer, 114:1 (1988); Bandak et al., “Polymer Performance in Sludge Conditioning”, Proceedings of the Eighteenth Mid-Atlantic Industrial Waste Conference, Lancaster, Pa. (1986); Doyle et al., “Sludge Conditioning With Organic Polyelectrolytes”, Proceed. of the Nat'l. Conf. on Municipal Treatment Plant Sludge Management, Orlando, Fla. (1986). Increased polymer usage increases both the disposal costs and the organic loading of the waste stream. The net result is that the amount and cost of sludge disposal can increase disproportionally when an existing plant must deal with increased loadings and clean-up requirements.
Recent research often purports to reduce waste volume by improved dewatering. See Knocke et al., “Effect of Mean Cell Residence Time and Particle Size Distribution on Activated Sludge Vacuum Dewatering Characteristics”, J. Water Pollution Control Fed., 58:1118 (1986); Barraclough et al., “Start-Up Optimization of the Mechanical and Chemical Parameters Influencing the Dewatering Performance of a Gravity Belt Filter Press Operation”, Proceedings of the Eighteenth Mid-Atlantic Industrial Waste Conference, Blacksburg, Va. (1986); Katsiris et al., “Bound Water Content of Biological Sludge in Relation to Filtration and Dewatering”, Water Res. 21:1319 (1987); Harries et al., “Design and Application of a Modern Solid/Liquid Separation Plant”, S. African Mech. Engin., 37:481 (1987); Cobb et al. “Optimizing Belt Press Performance at Smurfit Newsprint”, Tappi Proceedings—1987 Environmental Conference, Portland, Oreg. However, the pollution potential of the sludge is unchanged if such dewatering does not reduce the sludge's volatile fraction (“VS”).
Anaerobically digested sludges contain about 40%-75% VS. The VS content of undigested sludge is even higher. See Downing et al. “Used-Water Treatment Today and Tomorrow”, Ecological Aspects of Used-Water Treatment, Vol. 2, C. R. Curds and H. A. Hawkes, Eds. (1983); Ramalho, Intro. to Wastewater Treatment Processes, 2nd ed. New York: Academic Press (1983). Clearly, the potential for further reductions in sludge volume remains.
In general, other animal waste such as that from bovine, ovine, and chicken is treated in a similar manner. More recently, these sludges often are converted into fertilizer. However such conversion is not easy. For example, hog manure and urine contains or evolves into ammonia, hydrogen sulfide, methane, nitrates, trihalomethanes, spores of molds, and other contaminants. Animal effluvia and putrefactive gases from animal and vegetable tissue often exist in and around hog barns. Putrefaction produces highly odorous gases and compounds such as ammonia, amino acids, aromatic fatty acids, metabolites, mercaptans, and hydrogen sulfide. A waste management system must account for these odoriferous substances, and, more importantly, must remove viable noxious microbes such as fecal coliforms, including, or course Escherichia coli. 
A variety of techniques such as sonication are used to destroy noxious microbes as for example, described in U.S. Pat. No. 6,039,867. This patent patent describes using 700 to 1000 watts of sonication in “sonic packet” repetition and multiple passes. A side effect of sonication is heating. For example, the sonication described in U.S. Pat. No. 4,340,488 reportedly heated sludge to 70 degrees centigrade, which was described as a very desirable feature for killing microbes. Other sonication treatments may be found in U.S. Pat. Nos. 4,944,886; 4,477,357 and 5,380,445. A goal of sonication treatment is to kill as many microbes as possible, while heating the sludge with sonic energy. U.S. Pat. No. 5,380,445, for example emphasizes that 55 degrees centigrade is “the most effective” temperature for destruction.
Sonication also effects dissolved gases. While recognized as removing dissolved gases, this attribute has not been exploited generally by combination with other procedures to take advantage of gas control from the sonication itself. Another problem that is underappreciated or ignored is that microbe destruction by sonication as described in these patents is non-specific. Unfortunately in this regard, heterotrophic organisms often are desired and can assist sludge detoxification by removal of toxins and by competition with undesirable organisms. Finally, a major problem that blocks more extensive recycling of sludge as value added products such as fertilizer is the need to transport very wet materials to drying and pelleting stations. Still further most microbe killing schemes require extremely high energies and often use heat to kill microbes and to dry sludge for pelletization. Any method or system that can lower the energy costs, improve dewatering, and/or preferentially eliminate undesirable microbes can improve the cost structure for more efficient and more widespread use of sludge recycling.
Generally, sludge treatments described in the patent literature often represent scale up of laboratory methods and tools that, while suited for basic research often fails to account for large scale economies. More appropriate technology such as methods and tools for utilizing lagoons and large scale microbial conversion, often require exceedingly large resources of time, space and money to convert sludge into a form that can used, generally as fertilizer at a distant location.